Self adjusting pneumatic control apparatus



SELF ADJUSTING PNEUMATIC CONTROL APPARATUS Filed OGb. 8, 1954 42 4o 47N 4e 46 amr 45 INVENTOR. Robert C. Du Bois ATTORNEY.

nited States Patent i SELF ADIUSTING PNEUlVIATIC CONTROL APPARATUS Robert C. Du Bois, Fairfield, Conn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application October 8, 1954, Serial No. 461,102

15 Claims. (Cl. 137-86) A general object of the present invention is to provide an improved apparatus for regulating the magnitude of a variable. More specifically, the present invention is concerned with an automatic controller for regulating the magnitude of a variable with means being provided for automatically adjusting the controlling action of the controller to achieve a high degree of stability and a desired optimum control action in the overall control loop which includes the controller.

Automatic controllers for regulating the magnitude of some controlled variable have long been used in numerous types of processes. A typical type of automatic controller of the pneumatic type will be found in the patent to C. B. Moore, No. 2,125,081, issued July 26, 1938. This controller is of the type which incorporates means for varying the gain or proportional band of the controller as well as the reset action of the controller. The term reset action is here used to designate that action which ths controller takes to change its output in a variable correcting direction when the variable continuously deviates or droops from a predetermined set point. The proportional band and reset of the Moore type apparatus are adjusted manually to suit the particular process which is under control. Frequently, the variables aiecting the process may shift or change and one particular set of proportional band and reset adjustments will not always provide the optimum control action. Consequently, the operator generally adjusts the controller to a point where it will not be subject to instability under all expected process changes. This is accompanied by a loss of optimum control under certain operating conditions and hence the eiectiveness of the controller is lessened and the process operating efficiency is lessened.

The present invention is concerned with providing means which will automatically adjust the controlling action of the controller to a desired optimum operating condition. ln other words, the present invention illustrates a form of self-adjusting controller which automatically adjusts its controlling action in accordance with an indication of a deviation of the process from a desired operating state. This deviation may be represented by a steady state otset of the control variable from a desired set point value or by an oscillation of the control process.

The control adjusting component of the present invention is a variable gradient spring of the uid type. This spring is attached to the controller in such a manner as to eiect adjustment of the reset and the proportional band settings of the controller. A variable control uid pressure for the spring is developed by means indicative of an undesired deviation in the process or the controlling action of the controller.

lt is accordingly a more specific object of the present invention to provide an improved self adjusting controller which incorporates an automatically adjustable gradient spring for eiecting a change in the controlling action of the controller.

Another more specific object of the present invention is to provide an improved controller employing a uid j 2,169,453 rate-rated Nov. 6, s

spring which has a uid pressure supplied thereto in accordance with means indicative of an oscillatory condition in the control process or in accordance with the steady state offset of the control variable of the process from a desired set point value.

Still another object of the present invention is to provide a variable gradient spring for a controller having a follow up means and slow acting means for slowly removing the follow up eiect of the controller with the variable gradient spring connected to the controller to adjust the follow up action as well as the rate of removal of the follow up action.

A further object of the invention is to provide a fluid spring for a controller which is supplied with a pressure indicative of the amplitude of oscillation of a variable associated with the process.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and forming part of the specication. For a better understanding of the invention, its advantages, and speciiic objects attained with its use, reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

Of the drawings:

Fig. l represents a schematic showing of the pneumatic control system wherein proportional band and reset adjustments are accomplished in accordance with the principles of the present invention; and

Fig. 2 shows a modiiied form of a portion of the apparatus of Fig. 1 wherein only a proportional band adjustment is accomplished.

Referring first to Fig. l, the numeral 10 represents a furnace wherein it is desired to maintain a particular temperature condition. It should be noted that while the invention is explained with respect to a temperature condition, this is for illustrative purposes only and many variables may be controlled by the controller as will be readily apparent upon considering the discussion which follows. Sensing the temperature of the furnace 10 is a suitable thermal bulb 11 which is connected to supply a pressure to a suitable spiral 12. The end 13 of the spiral 12 is connected to a link 14 which leads to a suitable mechanism 15 which may be used to select the zero and set point of the controller of the apparatus. The form of the linkage and adjusting mechanism 15 may well be of the type disclosed in the above-mentioned Moore patent. They linkage 16 in the form of a bell crank is used to connect linkage 15 to the flapper member 17 at 18. The apper has cooperating therewith a suitable nozzle 19 which is supplied with uid under pressure by way of a conduit 26, thelatter having an adjustable restriction 21 in series therewith. The back pressure in the conduit 20 from the nozzle 19 is applied to a suitable pneumatic relay 25 which is supplied with air under pressure by a conduit 26. The output pressure of the relay 25 is fed to a conduit 27 to a suitable control valve 2S which is positioned in the uid ow line 3) which leads to the furnace 10.

The output pressure from the relay 25 is also fed through a conduit 32 to a chamber 33. This chamber 33 includes a bellows 34 which is sealed and has positioned therein a further spring loaded bellows 35. The space between the bellows 34 and 35 is lled with a liquid. Both of the bellows units 34 and 35 are sealed to a supporting member 36. The end of the bellows 37 which is opposite the supporting member 36 is rigidly attached to a rod 38, the latter of which is fastened at 39 to the iiapper member 17.

For producing an opposing force on the rod 38, thereV is provided a further pair of bellows units 40 and 41, the latter of which is inside of the bellows 40 and is spring 3 Y loaded. The leftend of both of the bellows units 40 and 41 are rigidly fastened to aV supportingimember 42 and the space between the bellows units'is filled with a liquid.

YThe space between the bellows units 34 and35 is conlthe Vpresence or absence of a deviation in the control process. f

" The processfdeviation sensing portion of the present invention includes a pivoted beam 55 which has a pair of diaphragm units 56 and 57 exerting opposed pressure forces on 'the beamr55. The pressure conditions for the diaphragms 56 and 57 are developed from the pressure condition in the conduit 27 Vand'V is fed to the diaphragms 56 and 57 by way of a conduit 553, a check valve unit 60,

Vand a further check valve unit 61 which `supply an air pressure to the chambers enclosed by the diaphragms 56 and 57 respectively. The chambers enclosed by the dia'- Vphragms 56 and 57 may be suitably inter-connected by a conduit 62 having an adjustable restriction 63 in series therewith. Y The beam 55 also has applied thereto a pressure force from a chamber which is enclosed by`a diaphragm unitV 65. This diaphragm receives its control pressure from a steady state deviation sensing mechanism 66. This mechanism includes a pivoted beam 67 which has applied to the end thereof a pressure force developed by aV spring unit 63 which is representative of the set point will be an increased pressure signal fed into the relay 25. The relayV 25 will also produce an increased pressure change which is to be fed to the conduit 27`to the valve 23 so that the valve 28Ywill open. When the pressure in Y the output of the relay 25 increases, the pressure is alsoV fedy through theconduit 32 to the chamber 33. YSince the chamber 33 is enclosed, the bellows 34 will tend to be compressedby the increased pressure in the chamber.V This compression force is Vtransferred to the bellows unit- 35 andV this bellows unit will tend to rnoV.e toward the` right and carry'with it the rod 3S. Movement ,of the.

rod 38 toward the right will'move the llapper17 away Y fromV the nozzle 19 and therebyV provide a follow-up action for the apper so as to reduce' the nozzle back pressure. The nozzleback pressure will accordingly stabilize Y out at some increased pressure value which will tend to conduit 32 to the chamber 33 where the reduced pressure causes the rod 38 to be moved to the left .and the flal'aperY 17 adjacentthe nozzle 19 to ybe moved in av pressure fin- Under normal conditions, the apcreasing direction. paratus will stabilize out at the starting point.

1f there should beV steady or continuous oiset between set point and variable` in the apparatus, it is desired that the controller reset the. output control pressure to the'` valve 23 in such a manner as to eliminate the oiset. The

' bellows units 40 and 41( provide a resetting YrriechanisruY of the process.` Opposing the spring 68 is a force developed by the diaphragm unit 69 which is supplied with 'Y a variable pressure by a conduit 7G which leads to the temperature sensing bulb 11.VV A pair of nozzles 71 and 72 la-re positioned on opposite sides of the beam 67 and are supplied with air by way of an input conduit "73 and anadjustable restriction 74.

The beam 55 -also has applied theretoa springV cle-` veloped force which is produced by an adjustable spring 75l. 'AV further force on the beam 55 is produced by a diaphragm 73 which encloses a chamber which is supplied by a pressure condition produced by the output relay Stil The relay 56 in turn receives its controlling pressure Vjfrom Ya nozzle 79 which cooperates with the beam 55 in a Vdapper-nozzle conguration to supply a variable pressure to the relay 50. Pressure is supplied to the nozzleA 79 by way of a conduit 80V and a variable restriction 81.

InV considering the operation Vof Fig. l, the operation will be considered rst'without the self adjusting controlV features. It is rst assumed that the apparatus is in aV balanced state Vas it is shown in Fig. ll. When in a balanced state, the temperature within the furnace 10l will be of the desired magnitude and the air pressure con'Y trolling the valve 28 will also be at s uch a magnitudeasY Y Vto maintain the proper amount of fuel ilow tothe furnace 10 to maintain'the temperature constant.

clockwise movement will be fed through the link 14,

zero mechanism 15, link 16 tothe flapper A175, and the apper 17 will be moved toward the right against the nozzle 19; Movingy the flappery 17 against the nozzle 19 will increase the'back pressure' in the nozzle 1'9 and there for the present controller and this mechanism is effective to slowly eliminate the rebalancing effect normally accomplished by the bellows units34 and 35. This rel setting effect is Vproduced by the pressure condition between the bellows 34 and 35 being dissipated Vinto the space between the bellows 40 and 41 by way of the passage 43 which has the restriction 44 therein. Thus, if there should "be an increase pressure condition in the chamber 33 which is continuous over a period ottime, Athe I initial` reaction to this pressure increase will be to de'-A ect the follow-up rod 38 toward the right.V However, as the pressure on` the liquid .between the bellows unitsV and 35 begins to dissipate into the space between thel bellows units 40 and 41, the force eXerted on the rod 38 which moves it towards the right will be decreased. Thus,

over a period of time, the rod 38 will always Vtendl to come back to a neutral position and. theV pressuresV acting upon the bellows 35 and 41'will tend to be balanced. `The rate at which ythis balancing takes Vplace between the follow up and the' reset bellows will be dependent 'uponV the 'amount ,ofresistanc'e or restriction presented by the" restriction 44. Y

IThe operation ofthe apparatus .as described thus far i has Vbeen yillustrative only of the'typev of operation ob'- tainable with apparatus Vof vthe t'ype disclosed inthe labovementioned Moore patent. Reference should be had to the MooreV patent for a more complete disclosure of the operation of the apparatus and the details relative tothe structure thereof.

As mentioned above, the presenti'app.a'ratus includes Y Y means for achieving .a self adjusting conguration in the,L

form of the fluid spring 45 which is supplied with a pres-'V decreased, the gradient' of the spring will be decreased.' Y

In'other Words, when the pressure ,applied to vdiaphragrns 46 and 47 is increased, the diaphragmsfpstretch outwardly freenetand cause the center member 49 to be more rmly held in its central position. When held in this manner, a greater force is required to displace the member 49. Thus, the stiiness or gradient of the spring is varied.

Before considering the specific functioning of the fluid spring 45, it should be noted that when there is an oscillation in the process as evidenced by an oscillation in the measured variable, or in the control pressure fed to the control valve, there is a need for decreasing the gain of the controller. The decreasing of the gain is sometimes referred to as a widening of the proportional band of the controller. In other words, when the gain of the controller is low, a unit change of magnitude in the input variable will produce a smaller output pressure change at the control valve than would be achieved if the gain of the controller were high. Further, when there is an oscillation present in the system, the oscillation may be eliminated by changing the reset rate. The direction of change is generally in a rate decreasing direction so as to prevent the apparatus from eliminating the follow up of the controller prematurely and instigating a further instability.

In the event that there is a steady state oiset, it is desired that the apparatus be changed in its controlling action to eliminate this offset. The elimination of a steady state offset is generally accomplished by increasing the reset rate and narrowing the proportioning band or increasing the gain of the controller.

The uid spring as used in Fig. l is eiective to change both the proportional band or gain and the reset rate of the controller. When the pressure within the spring is low, the gradient of this spring will be low and therefore will eXert substantially no restraining force upon the end of the bellows 40 as the bellows tend to move due to the controlling action of the controller. This will be eective to widen the proportional band of the controller and to decrease the reset rate. The widening of the proportional band will be apparent when it is noted that the follow up action upon the rod 38 by the bellows 35 is normally restrained by the bellows 41. If there is no substantialv restraining force upon the bellows 40 surrounding the bellows 41, the bellows 41 will be free to move toward the right so that there will be a relatively large follow-up action upon the ilapper 17. Insofar as the reset rate adjustment is concerned, it will again be noted that the ability of the bellows 41 to force the rod 38 back in a balance eliminating direction will be dependent upon how rigid the right end of the bellows 40 is maintained. If the bellows 40 is not restrained at its outer end, the bellows 41 will not be subjected to as large a pressure force tending to move the rod 38 in a balance removing direction. This has the effect of appearing as a decreased rate of reset action.

I f the gradient of the uid spring 45 is increased, the signal changes noted in the controlling action above are just reversed. In other words, the higher spring gradient will cause an increase in the rate of reset due to the fact that the bellows 41 is now constrained from movement at its right end and the pressure change is transferred to the bellows 41. Further, the proportional band or gain is narrowed which in eiect requires that there be a larger pressure change in the output for a unit change in input variable. y

The operation of the apparatus with its self adjusting features considered is first analyzed from the standpoint of an oscillatory condition in the process. An oscillatory condition in the process may be evidence either by an oscillation of the control variable as sensed by the bulb 11 or by an oscillatory condition of the pressure feeding the valve 28. The latter is selected in the present apparatus in that the amount of control signal available is generally larger than that available at the sensing element llandit is further possible to detect an instability at thevalve'28 prior to its becoming effective in the variable sensing element 11. With an oscillatory condition present in the conduit 27 feeding the valve 28, this oscillatory pressure will be fed throughthe conduit 59 to the check valve units 60 and 61. The high pressure signal will be passed through the check valve 60 while the low pressure will be passed through the check valve 61. This will establish on the diaphragm-s 56 and 57 a diierential pressure which is proportional to the amplitude of the oscillatory signal present in the line feeding the valve 28. There will be a resultant pressure force upon the beam 55 tending to rotate the beam in a counter-clockwise direction. This will mean that the beam 55 at its right end will move away from the nozzle 79 and there will be lowering of the back pressure in the nozzle and consequently a lowering of the pressure on the output of the relay 50. A rebalancing action will take place due to the action of the diaphragm 78 acting upon the beam 55. The decrease in pressure will also be applied by way of the conduit 83 to the fluid spring 45 so as 4to decrease the gradient of the spring as it acts upon the bellows 40. As pointed out above, the decrease in the spring gradient has the eiect of widening the proportional band and decreasing the rate of reset of the controller. This adjustment will return stability to the controller and to the process and the oscillation will cease.

In the event that there is a steady state olset or drooping of the control variable from the desired set point, it is desired that the proportional band be narrowed and the rate of reset be increased. As mentioned above, this may be accomplished -by increasing the spring gradient of the fluid spring 45. The oiset sensing mechanism 66 provides the pressure signal for effecting an increase pressure signal in the lluid spring 45. As long as the beam 67 of the apparatus 66 is centered between the nozzles 71 and 72, there will be a minimum low pressure applied to the diaphragm 65 associated with the beam 55. The beam 67 will be centered so long as the spring force 68 representing the set point is equal to and opposite the force presented by the diaphragm 69 which represents the magnitude of the control variable. When so centered, the air issuing from each of the nozzles 71 and 72 will be at a maximum value so that the pressure on diaphragm 65 will be low. If the magnitude of the variable should deviate above or below the desired value, the beam 67 will be moved upward or downward and there will be an accompanying increase in the pressure acting upon the diaphragm 65. This pressure force acts upon the beam 55 to rock the beam in a clockwise direction so that the right end of the beam moves adjacent the nozzle 79 and increases the back pressure in the nozzle 79. This increase in back pressure is fed through the relay 50 and the conduit 83 to the Huid spring 45 so as to increase the spring gradient thereof. As pointed out above, the increase in spring gradient increases the rate of reset and narrows the proportional band which means that the overall sensitivity of the apparatus will be increased and the oiset will be eliminated.

It will be readily apparent from the foregoing description that the apparatus of Fig. l is a self adjusting controller which adjusts both proportional band and reset so as to obtain optimum operation of the process controlled by the controller.

The apparatus shown in Fig. 2 is a modification of the Moore patent apparatus and is arranged so that the self adjustment is accomplished only with respect to the proportional band or gain of the controller. Corresponding components between Figs. l and 2 carry corresponding reference characters.

The fluid spring in this figure is identified by the numeral and it comprises a pair of diaphragm units 91 and 92 which correspond to the diaphragm units 46 and 47 of the fluid spring 45. The fluid spring 90 is coupled to the link 94 which is between the inner bellows units 35 and 41. The connection of these units to the spring 90 is at 95. The link 94 connects'to the flapper 17 inthe direction.

Vin Fig. l.

By placing 'the Vfluid Yspring 9i! directlyV on the connecting rod 94, it is Ypossible to exert a direct restraining force upon the member 94 as it normally tends to move Vthe apper 17 in at rebalancingY direction.

When the high pressurev signal is fed through the input conduit 3 3 to theuid spring 90, the gradient ofthe spring will bey increased and it will resist the forces acting Yupon the rod 94 tending to move Vthe apper in a balancing Th' has the eiect of narrowing the proportional bandV of the controller.

If the pressure fed to the liuid spring 99 is decreased, there will beY a smaller resistance force acting uponV the rod 94andfconsequently the proportional band Vof the controller will be widened.

The pressures supplied ,te the fluifl springe? maybe derived in the manner in which theyare derived in lig. lV orby any other suitable apparatusV which will indicate'arid differentiate betweenoscillatory instabilities and instability evidenced by a continuous oset.

While, in accordance with the provisions of the statutes, there has been illustrated and described the best forms of the invention known, it will be apparent to those skilled in the art that changes may be made in the form ofthe apparatus without departing from the spirit of the invention as set forth in the appended claims, and that in some cases certain features of the invention may be used to advantage Without a corresponding use of other features.

Havingnow'described the invention, what is claimed as new and for which it is desired to secure Letters Patent isr f 1. Apparatus for regulating a variable comprising a controller for producingVV an output control action which follows proportionally the magnitude of an input variable, controller sensitivity changing rmeans connected to s aid controller, said changing means comprising a pneumatic spring, ,and means responsive to lau aperiodic variation of the input variable connected to supply a able, controller sensitivity changing means connected to said controller, said changing means comprising a pneuspring, and means responsive to a periodic variation of the regulating apparatus to supply a pneumatic,

pressure to said Vfluid spring to of saidcontroller.

. 3. A controller for a .variable comprising means for producing an output control action proportional' to the magnitude of a variable, means for integrating a deviadecrease the sensitivity .tionjof .the variable from a predeterminedvalue and modifying the -output controlling action in accordance. with the integrating action of said means, a variable uid spring connected to said controller to regulate the integrating action of s aid means, and means indicative of a need for a changeY in therate of integration of said integrating means connected to supply a variable iluid pressure to said tluid spring.

4. Self adjusting mechanism for a controller comprising, a uid spring whose gradient is variable in accordance with Vthe magnitude or" an applied pressure, means connecting said spring to a controller -to modify the controlling action thereof, and means indicative of a process deviationconnected to supply a pressure signal to saidV spring which varies in accordance with the Spin a pneumatic controller, the kcomlnnation cornprising, a pneumatic followup element, aKV delayed action follow up cancelling element, an adjustable gradient springV connected to saidV follow up f cancellin'gelement tovay the effect of both said elements, and-'means indicative of of-a deviation in theictmtrollingY actionof the controller connected-to vary the gradient of said spring.

6. A self adjusting controller comprising, a apper- Vnozzle apparatus which is adjustable in .accordancewith the magnitude of a controlled variableto produce an output controlling pneumatic pressure, balancing means for the frapper-nozzle comprising a rst pneumaticmeans connected to adjust the dapper-nozzle Vrelation in afollow up direction, a second means 'slowly acting in opposition .to said nrst pneumatic means to rernove the balancing action of said iirst pneumatic means, a rigid connection between said iirst pneumatic means and said second means, and -a uid spring connectedY to exert a controllable restraining force on said rigid connection.

' 7. Apparatus as defined inclaim V6 wherein the gradient of said iluid spring is varied by a fluid pressure produced by means indicative of a deviation in a variable regulated by said controller. Y v

8. A self Vadjusting controller comprising, a appernozzle apparatus which is adjustable in accordance with the magnitude ot a controlled variable-to producean output controlling pneumatic pressure, balancing-means for the dapper-nozzle .comprising a first pneumatic means connected to adjust .the tiePne-,iiezzle relatieI1 iii e iellew up direction, a second means slowlyaillg .in opposition to said rst pneumatic means to Yremove the balancing action of said first pneumatic means, a iluid spring operatively arranged to eirert a controllable restraining force 011 seid seeetld mesas, and ineens eeilneetesl te 4seid limiti spring to vary the iluid pressuretherein in accordance.

with Ya deviation a variable regulated by Ysaid controller,

said fluid spring being operative to vary the follow up.-

action of said rst'pneulnatic means and the rate atiwhich said second means acts to remove'the balancing action ofA fsaid Viirst pneumatic means.

troller lfrom' a predetermined norm.

tude ofa deviation of the controlling action of saidlcon- 10. A self adjusting controller ,comprising fluid spring '1 connected tovary the controlling action of thecontroller,

and means for supplying apressure to said duid spring, said means comprising a dapper-nozzle apparatus having means for providing an increased output pressure to-said lluid springfirrespective of Ythe direction deviationof.V the controlled variable from'a predetermined set point.

l1. A self adjusting controller comprising" a iluid spring connected to varylthe lcontrolling action ofthe'zcontoller, and means for supplying apressure to said fluid spring,- said means comprising Ya vi'rst. dapper-nozzle apparatus havingY aV force applied. thereto VwhichV is .proportional to the amplitude o f adeyiation ofthecontrolling actionof series 'with the input of the'other of said pressure sensing units, means connecting botti ofsaidcheckyalves ,to ailuid pressure source indicativeot controller action, and'rneans 9 indicative of applied force to said beam to supply a controller adjusting signal.

13. Apparatus as dened in claim 12 wherein said pair of opposed pressure sensing units are connected by a passage having a controlled restriction therein.

14. Apparatus for regulating a variable comprising a proportional controller having balancing means and means for slowly removing the balancing action of the balancing means, mean-s responsive to a predetermined deviation in the operation of the controller, and a unitary controller adjusting means connected to said controller to simultane ously regulate vthe effect of said balancing means and said balance removing means, said unitary adjusting means being connected to said deviation responsive means and controlled thereby.

l() 15. Apparatus as defined in claim 14 wherein said unitary adjusting means comprises a uid spring.

References Cited in the le of this patent UNITED STATES PATENTS 2,125,081 Moore July 26, 1938 2,163,982 Mercier June 27, 1939 2,441,405 Fitch M-ay 11, 1948 2,452,176 Bent Oct. 26, 1948 2,638,921 Caldwell May 19, 1953 2,655,384 Peterson Oct. 13, 1953 

